Discrete time filters using switched-capacitor integrators are used in a variety of applications. One such use is in the analog loop filter in the modulator of a delta-sigma analog-to-digital converter.
Unfortunately the amplifiers in integrators produce flicker (1/f) noise and low frequency interference which, in the first stage of a loop filter of a delta-sigma converter, degrades the performance of the converter. One method used in the past to attenuate the flicker noise and low frequency interference in switched-capacitor filters is to chopper stabilize the amplifier in the integrator. See, for example, Hsieh, et al. "A Low-Noise Chopper-Stabilized Differential Switched-Capacitor Filtering Technique," IEEE Journal of Solid-State Circuits, Vol. SC-16, No. 6, pp. 708-715. In this article, a system is described for modulating any flicker noise and low frequency interference out of the bandwidth of interest by chopper stabilization.
In the circuit described by Hsieh, and in other similar discrete time circuits, the chopping or modulation frequency is limited to below the circuit's Nyquist rate, typically one-half the sampling frequency. In these circuits, chopping above this frequency is inefficient, since the noise will alias back down into the Nyquist band once it is sampled. However, in a delta-sigma converter, chopper stabilization of a discrete time converter which is performed at or below half the sampling rate is a potential tone generation mechanism. This tone generation mechanism is discussed in U.S. patent application entitled "Delta-Sigma Analog-to-Digital Converter With Chopper Stabilization at the Sampling Frequency," Ser. No. 07/429,211 filed on Oct. 27, 1989, now U.S. Pat. No. 5,039,989. This application is assigned to the assignee of the present application, and is hereby incorporated by reference. This application describes a circuit in which chopper stabilization is performed at the same frequency as the sampling rate and uses a continuous time integrator to filter the modulated noise before it is sampled.
However, there are various applications where an all discrete time loop filter has certain advantages over a continuous time filter, or a mixture of a continuous time and discrete time loop filter. Therefore, it can be appreciated that a switched-capacitor integrator with chopper stabilization performed at the sampling rate is desirable.